Generally, a semiconductor memory device is classified into a volatile memory device and a nonvolatile memory device depending on whether or not it is necessary to supply power for retaining stored data. The volatile memory device such as a dynamic random access memory (DRAM) and a static RAM (SRAM) has a rapid operational speed but there is a limitation in that it is necessary to supply the power for retaining the data. On the contrary, since the nonvolatile memory device such as a flash memory device does not have such a limitation, it is widely used for a portable electronic device that demands have dramatically increased in recent years.
For example, the flash memory device includes a memory cell for playing a role in storing data, and a device related thereto such as a select transistor, a drive transistor, or the like. The memory cell of the flash memory device mainly employs a memory transistor, which is similar to a typical transistor. The memory transistor includes a gate stack structure and impurity regions disposed on both sides of the gate stack structure, wherein the gate stack structure is configured with a tunnel insulating layer, a floating gate, a gate interlayer insulating layer, and a control gate which are sequentially stacked on a channel region of a substrate. The floating gates of adjacent memory transistors are electrically insulated to each other, and each floating gate acts as a memory. The control gates of a plurality of memory transistors arranged in row direction are interconnected so as to act as a word line. The flash memory device may be mainly classified into a NAND type one and a NOR type one according to the arrangement scheme of the memory transistor having the above structure. In case of the NAND type flash memory device, a select transistor is connected to the memory transistor, and for example, the gates of the select transistors arranged in row direction are interconnected to thereby form a select line.
Meanwhile, it is necessary to heighten the integration degree in order to reduce the price of the semiconductor device, which causes several technical difficulties in manufacturing the semiconductor device. In particular, as the semiconductor device is highly integrated, the space between adjacent word lines are reduced also, which makes it difficult to improve the structure and characteristic of the nonvolatile memory device. For instance, although the nonvolatile memory device having the control gate electrode and the floating gate electrode should have a high coupling ratio for an effective operation, the reduction of the space between the word lines makes it difficult to secure such a high coupling ratio.
In addition, since the width of the word line and the space between the adjacent word lines are smaller than the width of the select line and the space between the select line and word line, the active region of the substrate where the select lines will be formed may be etch-damaged due to a loading effect.
With the increase of the integration degree of the memory device, the channel length of the select transistor decreases also, which causes a short channel effect. For example, a punchthrough may occur with ease because a channel doping concentration is relatively higher in the edge portion than in the central portion of the channel region. Further, there is great likelihood that the punchthrough occurs at the memory transistor adjacent to the select transistor.